The present invention relates to low defect silicon carbide wafers and their use as precursors for semiconductor purposes, and to seeded sublimation growth of large, high-quality silicon carbide single crystals.
Silicon carbide has found use as semiconductor material for various electronic devices and purposes in recent years. Silicon carbide is especially useful due to its physical strength and high resistance to chemical attack. Silicon carbide also has excellent electronic properties, including radiation hardness, high breakdown field, a relatively wide band gap, high saturated electron drift velocity, high-temperature operation, and absorption and emission of high-energy photons in the blue, violet, and ultraviolet regions of the spectrum.
Single crystal silicon carbide is often produced by a seeded sublimation growth process. In a typical silicon carbide growth technique, the seed crystal and a source powder are both placed in a reaction crucible which is heated to the sublimation temperature of the source and in a manner that produces a thermal gradient between the source and the marginally cooler seed crystal. The thermal gradient encourages vapor phase movement of the materials from the source to the seed followed by condensation upon the seed and the resulting bulk crystal growth. The method is also referred to as physical vapor transport (PVT).
In a typical silicon carbide growth technique, the crucible is made of graphite and is heated by induction or resistance, with the relevant coils and insulation being placed to establish and control the desired thermal gradient. The source powder is silicon carbide, as is the seed. The crucible is oriented vertically, with the source powder in the lower portions and the seed positioned at the top, typically on the seed holder; see U.S. Pat. No. 4,866,005 (reissued as No. Re34,861) the contents of which are incorporated entirely herein by reference. These sources are exemplary, rather than limiting, descriptions of modern seeded sublimation growth techniques.
The invention is also related to the following copending and commonly assigned U.S. applications: U.S. Pat. No. 7,147,715 issued Dec. 12, 2006 for Growth of Ultra-High Purity Silicon Carbide Crystals in an Ambient Containing Hydrogen; U.S. Pat. No. 7,220,313 issued May 22, 2007 for Reducing Nitrogen Content in Silicon Carbide Crystals by Sublimation Growth in a Hydrogen-Containing Ambient; U.S. Pat. No. 7,033,912 issued Apr. 25, 2006 for Silicon Carbide on Diamond Substrates and Related Devices and Methods; U.S. Ser. No. 60/522,326 filed Sep. 15, 2004 for Seed Preparation for the Growth of High Quality Large Size Silicon Carbide Crystals; U.S. Pat. No. 7,192,482 issued Mar. 20, 2007 for Seed and Seedholder Combinations for High Quality Growth of Large Silicon Carbide Single Crystals; and U.S. Pat. No. 7,601,441 issued Oct. 13, 2009 for One Hundred Millimeter High Purity Semi-Insulating Single Crystal Silicon Carbide Wafer. The contents of these applications are likewise incorporated entirely herein by reference.
Although the density of structural defects in silicon carbide bulk crystals has been continually reduced in recent years, relatively high defect concentrations still appear and have been found to be difficult to eliminate, e.g. Nakamura et al., “Ultrahigh quality silicon carbide single crystals,” Nature, Vol. 430, Aug. 26, 2004, page 1009. These defects can cause significant problems in limiting the performance characteristics of devices made on the substrates, or in some cases can preclude useful devices altogether. Current seeded sublimation techniques for the production of large bulk single crystals of silicon carbide typically result in a higher than desired concentration of defects on the growing surface of the silicon carbide crystal. Higher concentrations of defects can cause significant problems in limiting the performance characteristics of devices made on the crystals, or substrates resulting from the crystals. For example, a typical micropipe defect density in some commercially available silicon carbide wafers can be on the order of 100 per square centimeter (cm−2). A megawatt device formed in silicon carbide, however, requires a defect free area on the order of 0.4 cm−2. Thus, increasing the quality of large single crystals that can be used to fabricate large surface area devices for high-voltage, high current applications remains a worthwhile goal.
Although small samples of low-defect silicon carbide have been available, a broader commercial use of silicon carbide requires larger samples, and in particular, larger wafers. By way of comparison, 100 mm (4″) silicon wafers have been commercially available since 1975 and 150 mm (6″) silicon wafers became available in 1981. Gallium arsenide (GaAs) is also commercially available in both 4″ and 6″ wafers. Thus, the commercial availability of 50 mm (2″) and 75 mm (3″) SiC wafers lags behind these other materials and to some extent limits the adoption and use of SiC in a wider range of devices and applications.
Screw dislocations, particularly 1 c screw dislocations, are common defects that develop or propagate during the production of SiC crystals. Other surface defects include threading dislocations, hexagonal voids, and micropipes. If these defects remain in the SiC crystal, then resulting devices grown on the crystal may incorporate these defects.
The nature and description of specific defects is generally well understood in the crystal growth art. In particular, a screw dislocation is defined as one in which the Burgers Vector is parallel to the direction vector. On an atomic scale, the resulting dislocation gives the general appearance of a spiral staircase. The presence of a large number of screw dislocations can also lead to the presence of other defects, such as micropipes and hexagonal voids.
A micropipe is a hollow core super-screw dislocation with its Burgers vector lying along the c-axis. Micropipes are often formed from a grouping of 3 or more screw dislocations. A number of causes have been proposed or identified for the generation of micropipes. These include excess materials such as silicon or carbon inclusions, extrinsic impurities such as metal deposits, boundary defects, and the movement or slippage of partial dislocations. See e.g. Powell et al., Growth of Low Micropipe Density SiC Wafers, Materials Science Forum, Vols. 338-340, pp 437-440 (2000).
Hexagonal voids are flat, hexagonal platelet-shaped cavities in the crystal that often have hollow tubes trailing beneath them. Some evidence shows that micropipes are associated with hexagonal voids. A relatively recent discussion of such defects (exemplary and not limiting) is set forth in Kuhr et al., Hexagonal Voids And The Formation Of Micropipes During SiC Sublimation Growth, Journal of Applied Physics, Volume 89, No. 8, page 4625 (April 2001).
The presence of surface defects in bulk single crystals of SiC may also interfere with single-polytype crystal growth. The 150 available polytypes of SiC raise a particular difficulty. Many of these polytypes are very similar, often separated only by small thermodynamic differences. Maintaining the desired polytype identity throughout the crystal is only one difficulty in growing SiC crystals of large sizes in a seeded sublimation system. When surface defects are present, there is not enough polytype information on the crystal surface for depositing layers to maintain the desired polytype. Polytype changes on the surface of the growing crystal result in the formation of even more surface defects.
Recent research indicates that problems in the bulk crystals produced in a seeded sublimation technique can originate with the seed itself and the manner in which it is physically handled; e.g., Sanchez et al Formation Of Thermal Decomposition Cavities In Physical Vapor Transport Of Silicon Carbide, Journal of Electronic Materials, Volume 29, No. 3, page 347 (2000). Sanchez uses the term “micropipe” to describe, “approximately cylindrical voids with diameters in the range of 0.1 μm to 5 μm that form at the core of superscrew dislocations aligned parallel or nearly parallel to the [0001] axis” Id. at 347. Sanchez refers to larger voids (“diameters from 5 μm to 100 μm”) as, “thermal decomposition cavities,” and opines that micropipes and thermal decomposition cavities arise from different causes. Id.
Accordingly, producing larger high quality bulk single crystals of silicon carbide with low 1 c screw dislocation defect levels in crystals formed in the seeded sublimation system, in order to reduce the total number of defects in the produced crystals remains a constant technical and commercial goal.